The production of electrochemical cells as described in GB2380055 involves polymerisation of ionically conducting material in situ, in a space or cavity. In order that the resulting structure should have the requisite electrical properties, there is commonly a need for the presence of a catalyst in, on or adjacent to each surface of the ionically active material.
In conventional electrochemical structures (e.g. fuel cells and electrolysers) and photo-electrochemical devices (e.g. photovoltaic, direct dissociation and photo-augmented electrochemical cells), where the ionically active material is either (i) in the form of a thermoplastic membrane (such as Nafion) or (ii) a liquid lye (acid or alkaline), the catalyst is usually introduced in one of two ways, as follows:
(i) The catalyst is provided in the form of finely divided particles which are separate or, in the case of a supported catalyst, attached to some matrix material (typically carbon particles or carbon paper when the carbon paper may additionally serve as the electrode). In this form, the catalyst is distributed uniformly over the surface of the membrane and pressed under the action of heat and pressure (for Nafion, typically 140° C. and 690 kPa) into the surface of the thermoplastic membrane. The objective is that the catalyst forms a uniform layer in intimate contact with the ionomer but accessible to the action of the reactant adjacent to the membrane surface, as shown in FIG. 1.
(ii) The catalyst is prepared as a layer on a surface defining the electrochemical cell. For example, in the case of a photovoltaic cell, the cell is constructed of two glass sheets coated with a transparent electrode (tin oxide) which is further coated with a catalyst (titanium dioxide to form one electrode and platinum to form the second) in the form of an insoluble layer attached to the glass/tin oxide surface. The resulting surface is then exposed to the liquid lye to complete the cell.
Problems associated with the use of these systems are well known. The solid polymer Nafion is inevitably dry during the deposition of the catalyst due to the temperature needed to render the Nafion soft and deformable, but in order to be an effective ionic conductor the Nafion must be hydrated. During this process the material swells and tends to delaminate, disrupting precisely the contact needed between the polymer and the catalyst. The use of liquid conduction materials in the form of a lye is also well known, but the use of a liquid imposes constraints upon the design and operation of the cell.